Cell/cell communication controls many aspects of cellular physiology including cell proliferation, cell differentiation and cell death/survival. However, the complexity of multi-cellular organisms has made it difficult to obtain a comprehensive understanding of all extracellular signaling mechanisms controlling these aspects. This research project focuses on the control of cell survival by extracellular, or non-autonomous, signaling. We have identified mutants in tumor-suppressor-like genes that control the secretion of extra-cellular factors which promote the survival of neighboring cells. These studies reveal interactions between cells which are very relevant for tissue homeostasis, and abnormalities may be directly linked to the parthenogenesis of human diseases including cancer. For example, animals containing mutant clones of these tumor suppressor-like genes are characterized by tissue overgrowth and tumor-like masses. In some of these mutants, Notch activity is inappropriately activated which stimulates proliferation and survival in a non-autonomous manner. Inappropriate Notch activation has been implicated for the genesis of many human cancers. Our data demonstrate that cell proliferation is not sufficient for generation of the tumor masses;instead increased cell survival is necessary for full development of tumors. Therefore, an understanding of the genetic and molecular mechanisms that control non-autonomous cell survival is crucial for the prevention and treatment of these diseases. It is the main goal of this proposal to further our understanding about the mechanisms that regulate non-autonomous survival. For this purpose, we are using the highly accessible genetic model organism Drosophila melanogaster. Our specific aims are: 1. Identify the genes in the signal-sending cell that control non-autonomous cell survival. 2. Identify the mechanisms which lead to secretion of signaling molecules that control cell survival in neighboring cells. 3. Identify the signals and the mechanisms in the signal-receiving cell that control non-autonomous survival. This project will be the first systematic approach to identify all genes and mechanisms that control non- autonomous survival in any organism. The characterization of these genes may have significant implications for the understanding of human diseases, and may help developing drugs and therapies to treat these diseases.